Multiferroicity from charge ordering? A case study
Manuel Angst
Peter Grünberg Institut und Jülich Centre for Neutron Science, JARA-FIT, Forschungszentrum Jülich GmbH,Jülich, Germany
E-mail: m.angst@fz-juelich.de
Magnetoelectric multiferroics have a large applications potential and often involve complex phase competitions. Among different possible mechanisms of multiferroicity, ferroelectricity originating from charge ordering is particularly intriguing because it potentially combines large electric polarizations with strong magnetoelectric couplings – but example materials where this is realized seem to be very difficult to find. After LuFe2O4 had been proposed to be a multiferroic due to ferroelectricity originating from Fe2+/Fe3+ charge order in 2005 [1], this material has become the generally accepted prototypical example of this mechanism and has correspondingly attracted increasing attention. The proposal had been made due to indications of a polar state by dielectric and pyroelectric measurements, and a reasonable model of charge order based on the location of superstructure reflections by X-ray diffraction. This charge order model has not been completely verified though [2], and the spin-order has also been a largely open question. I will present recent x-ray and neutron diffraction, and circular dichroism, measurements that now allowed i) a full refinement of the charge-ordered crystal structure [3], ii) the determination of the spin structures in two competing magnetic phases that are nearly degenerate at TN=240 K [4], and iii) the relation between these orderings [3].
The results reveal a very strong coupling between spin- and charge order even in the precursor region to 3D long-range order, but also strong oxygen-stoichiometry dependences of all properties. Most importantly, the unambiguously determined arrangement of Fe2+ and Fe3+
ions excludes any charger-order-based ferroelectricity, implying that a clear example material for this mechanism has yet to be identified. I will also briefly discuss very recent dielectric spectroscopy results [5] that attribute the original macroscopic indications of ferroelectricity to artifacts, consistent with the microscopic findings.
References
[1] N. Ikeda et al., Nature 436, 1136 (2005).
[2] M. Angst et al., Phys. Rev. Lett. 101, 227601 (2008).
[3] J. de Groot, T. Mueller, R. A. Rosenberg, D. J. Keavney, Z. Islam, J.-W. Kim, and M.
Angst, Phys. Rev. Lett. 108, 187601 (2012).
[4] J. de Groot, K. Marty, M. D. Lumsden, A. D. Christianson, S. E. Nagler, S. Adiga, W. J.
H. Borghols, K. Schmalzl, Z. Yamani, S. R. Bland, R. de Souza, U. Staub, W. Schweika, Y. Su, and M. Angst, Phys. Rev. Lett. 108, 037206 (2012).
[5] D. Niermann, F. Waschkowski, J. de Groot, M. Angst, and J. Hemberger, Phys. Rev.
Lett. 109, 016405 (2012).
